At present time, silicon is mainly used as the material of the powder device; however as the material capable of realizing the power device with even higher performances, the wide band gap semiconductors such as gallium nitride or silicon carbide or so have attracted attention. Aluminum nitride has even wider band gap energy (6.2 eV) compared to these materials, and also have better dielectric breakdown voltage (12 MV/cm), and better thermal conductivity (3.0 Wcm−1K−1), hence aluminum nitride is capable of realizing a power device having even higher withstand voltage characteristic than in case of using the above mentioned materials.
For the device structure using gallium nitride and silicon carbide which attains high element performance (a large capacity, and a high withstand voltage), the vertical type semiconductor structure with conductive substrate as the supporting substrate, that is the electric current flows or the voltage is applied between the surface of the device and the backside is proposed (the patent articles 1 to 3). By employing the vertical type structure, the withstand voltage characteristic can be improved which was the object of the horizontal type device wherein the flowing direction of the electric current for driving the device is in the horizontal direction. Further, by employing the vertical type structure, a larger electric power can be applied to the device. Also, as the device which is effective to have the vertical structure, the luminescence device such as semiconductor laser or so may be mentioned. By employing the vertical type device structure, the electric current concentrating at the edge of mesa structure which is the problem of the horizontal type structure can be avoided, and a uniform electric current can be introduced to an active layer, thus the reliability of the device can be improved.
In order to realize such vertical type element structure, as mentioned in the above, the conductive substrate must be used. In regards with aluminum nitride having the conductivity, the method of forming the n-type conductive crystal layer by Si doping using metalorganic vapor phase epitaxy (MOVPE) method or a hydride vapor phase epitaxy (HVPE) method or so is known (the patent articles 4 to 6).
However, in the patent articles 4, 5 and 6, the n-type aluminum nitride single crystal layer is formed on the foreign substrate such as sapphire substrate or SiC substrate; hence it was difficult to produce the n-type aluminum nitride single crystal layer having high quality and a thickness sufficient enough to be used as the substrate.
On the other hand, as the method for forming a high quality aluminum nitride single crystal layer, the method which grows the aluminum nitride single crystal layer on the base substrate formed from same material that is the substrate formed from the aluminum nitride single crystal is also developed (the non-patent article 1). According to the method described in the non-patent article 1, a highly pure aluminum nitride thick film layer having the crystalline quality as same as the aluminum nitride single crystal base substrate can be formed. Further, according to this method, the n-type aluminum nitride single crystal with dislocation density of 106 cm−2 or so can be obtained.
Also, the example of forming the thick film of aluminum nitride on aluminum nitride base substrate by the chemical vapor phase growth is also described in the patent article 7 and in the non-patent article 2.
Also, as the method for stably growing the group III nitride single crystal having a low dislocation density and a good crystallinity, a physical vapor phase growth method such as a sublimation method or so is known. According to the sublimation method, a thick group III nitride single crystal can be obtained. The patent article 8 describes to carry out the sublimation under the presence of the impurity elements such as Si or so in order to increase the growth rate of the group III nitride single crystal and to improve the crystallinity.